Phosphate treatments are widely used in general as a temporary anticorrosion treatment for iron and steel, as a paint undercoating treatment for iron and steel (including zinc-plated iron and steel) and aluminum, as a lubricant undercoating treatment in the plastic working of iron and steel, and as a lubrication treatment for sliding parts. Phosphate treatments are used for these applications because phosphate coatings, which function as passivating coatings, have the ability to impart corrosion resistance to metals and because these coatings have an excellent affinity for organic chemical substances (e.g., resins and oils) and as a result support and enable excellent adherence between organic chemical substances and metal surfaces. In other words, phosphate coatings have the most essential properties required of a surface treatment coating: corrosion resistance and adherence.
Phosphate coatings occur in a variety of types, such as iron phosphate, zinc phosphate, zinc iron phosphate, zinc calcium phosphate, and manganese phosphate, as a function of the nature of the particular metal workpiece. While each of these coating types is used as appropriate based on its specific properties, the highest demand is for the formation of zinc phosphate coatings and zinc iron phosphate coatings on iron and steel, including zinc-plated iron and steel (composite coatings of zinc phosphate and zinc iron phosphate are usually formed on iron and steel surfaces).
The phosphate treatment baths used with iron and steel take the form of acidic aqueous solutions made up from phosphoric acid, nitric acid, and zinc as essential components along with various additives. A conversion coating is formed when, for example, iron or steel is brought into contact with such a bath for several minutes. Some of the elementary chemical reactions that are believed to occur during such contact can be exemplified by the following chemical reaction (or half reaction) equations (1) through (5):Fe−Fe2+2e−  (1)2H++2e−−H2  (2)3Zn2++6H2PO4−−Zn3(PO4)2.4H2O+4H3PO4  (3)2Zn2++Fe2++6H2PO4−−Zn2Fe(PO4)2.4H2O+4H3PO4  (3′)Fe2+−Fe3++e−  (4)Fe3++H2PO4−−FePO4+2H+.  (5)
Iron and steel dissolve according to equation (1) in acidic treatment baths such as phosphate treatment baths, and the electrons given up at this point are consumed in the discharge of hydrogen ions as in equation (2), causing an increase in pH at the metal surface. This increase in pH results in a shift in the degree of dissociation at equilibrium of the phosphoric acid, resulting in the insolubilization of a portion of the ferrous ions dissolved from the substrate and/or the zinc ions present in the phosphate treatment bath and formation of a coating of zinc phosphate and/or zinc iron phosphate on the substrate surface according to equation (3) and/or (3′).
While the primary driving force for these coating-forming reactions is dissolution of the substrate according to equation (1), a large fraction of the dissolving ferrous ions ends up unused by the reactions. These “waste” ferrous ions must be removed from the <system, since they hinder diffusion of the zinc and phosphate ions and thereby lower the coating-forming reaction rate. In general, the ferrous ions are oxidized to ferric ions according to equation (4) by an oxidizer additive such as nitrite ions and precipitate as insoluble iron phosphate according to equation (5).
The ability of this chemical reaction system to eliminate the evolved impurities from the system as a solid precipitate enables use of the treatment bath on a semi-permanent basis simply by replenishing the consumed components—a feature that has contributed greatly to the industrial and commercial success of phosphate treatments. This notwithstanding, removal of this hydrous solid (sludge) requires complex management sequences, while the cost of treating the discharged sludge, which is an industrial waste, has been increasing. These factors have recently led to stronger demand specifically for a nonsludging phosphate treatment.
The execution of phosphate treatment using cathodic electrolysis is one counter-measure to the sludge problem. Cathodic electrolysis differs from the above-described conversion-based phosphate treatment in that reaction (2) is driven in cathodic electrolysis directly by electrical energy from an outside power source. The substrate dissolution reaction (1) is no longer necessary and the production of iron phosphate sludge can be avoided. However, since sludge actually also contains about 10 to 25% zinc phosphate in addition to iron phosphate, the use of just cathodic electrolysis cannot completely eliminate sludge production.
A number of processes for carrying out phosphate treatment by cathodic electrolysis have in fact already been disclosed in the prior art, most prominently in Japanese Laid Open (Kokai or Unexamined) Patent Application Numbers Sho 64-21095 (21,095/1989) and Hei 4-36498 (36,498/1992) and Japanese Laid Open Patent Application (PCT) Number Hei 6-506263 (506,263/1994). The object of Japanese Laid Open (Kokai or Unexamined) Patent Application Number Sho 64-21095 is high corrosion resistance and high adherence in application as a paint undercoating. This process cannot avoid sludge production, however, because trivalent iron cations are present in its treatment bath. Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 4-36498 employs a high zinc-to-phosphoric acid ratio, probably because its object is the rapid formation of a fine and dense zinc phosphate coating. It is believed that a zinc phosphate sludge will be produced under these conditions. Japanese Laid Open Patent Application (PCT) Number Hei 6-506263, being concerned with countermeasures to the expense and toxicity of the nickel and/or cobalt sometimes deemed essential to the maximal performance of phosphate coatings as paint undercoatings, states that the concentration of these species in the treatment bath can be reduced through the use of electrolysis. Thus, no distinctive features can be discerned when the treatment bath compositions used in conversion processes are compared with these teachings; rather fine-sizing and densification (high corrosion resistance) of the coating or rapid coating formation is identified in each case as the advantage to the use of electrolysis and these teachings are silent on the subject of reducing sludge production.
The prior phosphate treatment technology as described above is thus unable to entirely eliminate sludge production. It is therefore an object of this invention to introduce a zinc phosphate treatment bath that is entirely free of sludge production. Another object of this invention is to introduce a zinc phosphate treatment process that uses said nonsludging zinc phosphate treatment bath.